Person A is a modern person who was transported to a parallel world where people are still in the middle ages. Person A assumes that the present Earth is round and he explains to person B that it is a magnetic sphere with a north pole and a south pole. Person B neither knows about this nor believes in it.
Person B: 世界は玉じゃないよ
Person A: ...もう1周まわって そっちが真実かもしれんな
The conversation ends here.
Now, I can't make sense of the last sentence. Who possibly could do the 回る here?